Neuronal Source of Plasma Dihydroxyphenylalanine

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Abstract
The source and significance of plasma levels of dihydroxyphenylalanine (DOPA), the precursor of the endoge-nous catecholamines, have been unknown. We measured arterial and venous plasma DOPA concentrations in healthy subjects at rest, patients who had undergone regional sympathectomies or were undergoing general anesthesia, and subjects during procedures (tilt, oral clonidine, or iv isoproterenol, yohimbine, tri-methaphan, or diazepam) known to affect plasma norepineph-rine levels. We also measured plasma DOPA in laboratory animals during anesthesia, after adrenalectomy, or after administration of α-methyl-para-tyrosine, which competitively inhibits tyrosine hydroxylase, the intraneuronal enzyme catalyzing the rate-limiting step in catecholamine biosynthesis. In virtually all healthy subjects there was an arteriovenous increment in plasma DOPA (mean increase, 32%; P < 0.001), whereas in sympathectomized patients there was not (mean decrease, 16%; P < 0.001 compared with healthy subjects). Except for small decreases after clonidine treatment, none of the above procedures affected plasma DOPA levels. Plasma DOPA decreased during general anesthesia and returned to baseline upon reversal of the anesthesia. Adrenalectomy had no effect on plasma DOPA. α-Methyl-para-tyrosine decreased plasma DOPA by 62% (P < 0.01). The results support the suggestion that DOPA can pass across sympathetic neuronal membranes to reach the general circulation. If so, then the regional rate of appearance of DOPA in plasma may be related to the regional rate of tyrosine hydroxylation. Conversely, DOPA taken up from the circulation may provide a source for catecholamine biosynthesis in tissues devoid of tyrosine hydroxylase.